The present invention relates to cardiac pacemaker systems and, in particular to such systems having the capability of indicating when the implanted pacemaker should be replaced due to battery depletion.
Implantable cardiac pacemakers are powered by batteries that have an initial energy capacity, which is a first and primary determinant of effective pacemaker lifetime. For example, lithium iodide batteries used in implantable pacemakers manufactured by Vitatron Medical, B.V., the assignee of this invention, have capacities varying from 600 mAh up to 1800 mAh. Starting with the battery capacity at beginning of life (BOL), the anticipated duration to effective end of life (EOL) is also a function of the operating conditions, e.g., the parameters of the pace pulses being delivered and the effective output load, or lead resistance. It is, of course, important to know when the battery is almost depleted, or empty, in order to safely replace the implanted pacemaker. As is known, an implantable pacemaker does not provide the ability to replace only the battery, which is sealed within, but rather the entire pacemaker must be replaced. Thus, it has been a longstanding concern of the pacemaker industry to provide an accurate indication of the battery status, and an accurate prediction of when the pacemaker is going to need to be replaced.
A number of techniques have been used in pacemakers for determining the status of the battery and indicating when the pacemaker is essentially at EOL and needs to be replaced. For example, it is known in the prior art to measure the battery impedance, which is well known for various lithium iodide battery models. The characteristic of battery impedance as a function of energy used, i.e., depletion level, is available in all cases, and is an accurate indication of energy used, and consequently of remaining available energy. However, the predictive value of the impedance measurement depends upon knowing the actual functioning conditions, i.e., ongoing current consumption. Thus, while an accurate impedance measurement can indicate the remaining energy, the remaining useful time is dependent upon operating conditions.
Another approach, which may be used alone or in combination with the impedance measurement, is that of a hardware comparator which compares the battery voltage with a reference voltage set to represent the voltage at which EOL is declared. Such a subsystem is substantially completely hardware driven, which has certain advantages. The battery voltage measurement also has an accuracy advantage, since it measures the actual operating parameter which defines when the pacemaker is operating within tolerance. Such a system guards that if the output voltage drops below the reference level corresponding to recommended replacement time (RRT), a flag is set, and the pacemaker can be automatically switched to a less energy-consuming mode of operation. However, the voltage measurement system is not so accurate a predictor of when RRT will come, meaning that the physician does not get an early warning of when to bring the patient back for pacemaker replacement.
Another prior art approach is that represented by U.S. patent application Ser. No. 08/132,713, filed Oct. 6, 1993, now U.S. Pat. No. 5,458,624 and assigned to the same assignee as this invention. The approach of this system is to provide a circuit for constantly obtaining a measure of the total battery current expenditure to determine battery depletion, rather than measuring battery parameters. Based upon a determination of what percentage of the battery capacity has been expended, and using the current rate of energy consumption, the physician can then project an effective lifetime for the pacemaker. This basic concept of projecting EOL by accumulating a measure of energy usage in an implanted pacemaker is also illustrated in U.S. Pat. Nos. 4,556,061 and 4,715,381. Such systems in practice require significant additional hardware and software in order to achieve their accuracy.
So long as the impedance measuring system can reliably provide sufficient advanced warning, it remains a recommended system due to its good predictability and its relative simplicity. While a voltage measurement system characterizes the present state of the battery within its functioning mode, and thus can signal actual RRT with a high accuracy, the overriding need of the physician is accurate predictability of when the implanted pacemaker needs to be replaced. These considerations indicate that a simple impedance measurement system is an optimal design choice if it can be adapted to account for changed current consumption conditions. However, since the predictability is a function of ongoing current consumption, the impedance measurement system must be supplemented by a capability to adapt the RRT determination in accordance with changed pacing conditions.
In the design of the system of this invention, we define three points in time which present important requirements which must be met. These requirements are defined in terms of tolerated drop of the programmed value of the output pulse. At RRT, the recommended replacement time, the output voltage must be greater than 80% of the programmed value. At last replacement time (LRT), the output voltage must still be at least 60% of the programmed value. Further, the time period between RRT and LRT, where the pacemaker is operating at standard conditions, e.g., 4.0 volts, must be three months for 99.7% of the population (3 sigma). The "Aging time" is defined as that time where, at standard conditions, the pacemaker has at least 6 months remaining until RRT, for 99.7% of the population (3 sigma). By providing an indication of Aging time, the physician can plan for a patient visit safely ahead of LRT, and can minimize RRT operation. These times are exemplary for purposes of this preferred embodiment, and it is to be understood that they can be defined differently.
By using battery tables available, the battery impedance value for RRT and Aging, can be determined for standard pacemaker operating conditions. This invention further provides means for adapting determination of RRT and Aging when any other pacing condition is programmed.